Re: [boost] Numeric constants

Subject: Re: [boost] Numeric constants
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-01-30 10:23:34

John Maddock wrote:
> template <class T> T pi();

I have a fixed-point class.  What actually happens when I call 
pi<fixed>() ?  Presumably the implementation just does "return 
3.1415...", so fixed::fixed(double) gets called.  Yes my fixed class 
has a constructor that takes a double, and if I'm really lucky it may 
do the conversion at compile time - but more likely it will happen at 
run time.  I would get the same code if you just declared "const double 
pi = 3.1415" (or #defined it).

My 64-bit fixed point type can store more digits of PI than a 64-bit 
floating point value.  But I'm not aware of any way of filling those 
extra digits [ignoring the existence of long double].  Similarly if I 
had an arbitrary precision (i.e. variable length) type, how would it be 
constructed from the 100-digit constants that you mention without 
support from the language (or maybe macros)?

(As for the actual constants, the only ones I find myself using are pi 
and the approximate radius of the earth.)

Cheers,  Phil.